1. Field of the Invention
This invention pertains to tanks or containers for storing liquids. Specifically, the invention refers to double-wall underground storage tanks.
2. Related Art
Underground storage tanks are commonly used to store liquids such as gasoline and other petroleum products. Underground storage tanks are commonly manufactured using steel or fiber reinforced plastic (FRP). There are problems associated with tanks manufactured with both of these materials. Steel tanks rust and pit. FRP tanks may crack if improperly installed. When environmentally harmful materials such as petroleum products are stored in these tanks, leaks are of great concern. This concern has led some governmental authorities throughout the world, including the U.S. and Canada, to pass legislation requiring the use of double-walled tanks, which provide secondary containment.
A typical double walled tank 100 is shown in FIG. 1. The tank 100 includes an outer wall 10 and an inner wall 20. The outer wall 10 and the inner wall 20 are in a closely spaced adjacent relationship to form an annular space 30 between the outer and inner walls 10, 20. The outer and inner walls 10, 20 are cylindrical in the central portion of the tank 100. The annular space 30 is air-tight and liquid-tight. A monitoring tube 40, which is in fluid communication with the annular space 30, extends to the surface so that the annular space 30 may be monitored. Located between the outer wall 10 and the inner wall 20 are a plurality of reinforcing ribs 50. The reinforcing ribs 50 are attached to both the outer wall 10 and the inner wall 20. The reinforcing ribs 50 provide structural support for the tank 100. A hollow tube 60 passes through each rib 50 at both the top and bottom (and other locations if desired) of the tank 100. The hollow tube 60 ensures that the spaces between each of the ribs 50 are in fluid communication with each other to form a continuous annular space 30.
The annular space 30 is monitored to detect leaks in the outer wall 10 and the inner wall 20. Several different schemes may be used to monitor the annular space 30, including wet, dry, pressure and vacuum. Such schemes are well known in the art. Wet alarm systems are the most common. In a wet alarm system, the annular space 30 is filled with a leak detection fluid such as brine. In such a system, the brine fills the annular space 30 and a portion of the monitoring tube 40. If a leak occurs in the outer wall 10 or the inner wall 20, the level of fluid in the monitoring tube 40 will drop. Thus, leaks can be detected by monitoring the level of the fluid in the monitoring tube 40.
Known alarm systems, such as the wet alarm system described above, are adequate for detecting the presence of leaks. However, such alarm systems do not indicate the location of a leak. If the tank 100 does not have a manway, or a leak occurs in the outer wall 10, it may be necessary to dig up and expose the entire tank 100 to determine the location of the leak. This process is expensive and time consuming. Great savings could be realized if it were possible to identify the location of the leak without the need to remove the tank from the ground. Knowing the location of the leak would allow the leak to be repaired without incurring the costs associated with exposing the entire tank.
What is needed is a double-walled, underground storage tank that can detect both the presence and location of a leak.
The aforementioned need is met to a great extent by the present invention which provides a ribbed, double walled storage tank with a plurality of annular spaces between the ribs. The annular space between each pair of ribs is connected to other annular spaces only at the top of the tank. By connecting the annular spaces between the ribs only at the top and pitching the tank as described above, the tank will fill one annular space at a time in a cascading fashion. When a leak occurs in either the outer or inner wall, the fluid in the annular space corresponding to the leak will partially drain to the level of the leak while the other annular spaces will remain nearly full. Thus, the leak location can be determined by simply examining the annular spaces between pairs of ribs, starting at the lowest pair and working up, to detect the first annular space that is not nearly filled with leak detection fluid. The leak will be located between the pair of ribs corresponding to that annular space.
Examining the annular spaces may be accomplished in several different ways. A first preferred way to locate the location of the leak begins by gaining access to the interior of the tank. This may be accomplished through an existing manway, or may require cutting a hole in the tank, which is sometimes done through the top of the tank and sometimes done through an endwall or dome. (The tank is normally pumped out, cleaned and the air in the tank tested for safety prior to entering the tank). Once someone is in the tank, the annular space is filled (or refilled) to the extent possible with monitoring fluid. A leak in an inner wall should be visually detectable. If the leak is in an outer wall, one simply taps on the inside of the tank between each pair of ribs until a hollow section is found. The leak is then known to be on one side or the other between the rib pair, and at a height corresponding to the level to which the monitoring fluid has fallen. One way to determine the correct side is to add pressurized air to the annular space and listen for air escaping the outer wall. Of course, one could simply guess as to one side as well, with the knowledge that the other side will also require repairing if the guess is wrong. The inside wall is then cut away, preferably in a section approximately one foot wide, centered at the suspected height of the leak and extending across the rib pair. The break in the outer wall is then located and repaired. Then the inner wall and any hole made in order to gain access to the inside of the tank are repaired and the tank is ready for further use.
A second way to locate the leak is to expose the top of the tank and tap on it to locate the hollow annular space between a pair of ribs corresponding to the leak. This method locates the leak between one pair of ribs. Then the tank is exposed only between that pair of ribs and a process that is the analogue of the first (repair from inside) method is employed.
A third way is to provide a separate alarm at or near the top of each annular space, which also has the benefit of allowing the approximate location of the leak to be detected without exposing any portion of the tank. The method may be used with any type of alarm system. For example, with a wet alarm system, the leak detection fluid is already present and detecting the leak location is performed as described above. For dry types of systems in which the annular space is normally not filled with leak detection fluid, fluid can be introduced after the leak has been detected.